Air carrier takes evasive action as TCASII RA indicates a climb, in IMC.

Attributes
ACN	237997
Time
Date	199303
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sat
State Reference	TX
Altitude	msl bound lower : 3000 msl bound upper : 3500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sat
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	climbout : intermediate altitude
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : flight engineer pilot : commercial pilot : cfi pilot : atp
Experience	flight time last 90 days : 180 flight time total : 4100 flight time type : 400
ASRS Report	237997
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 10000 flight time type : 150
ASRS Report	237053
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 0 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While being radar vectored for ILS approach, VFR traffic 500 ft below our aircraft was not reported. Our aircraft (large transport) was IMC, the single engine aircraft was just below overcast. Apparently single engine aircraft drifted up causing TCASII to compute a possible collision. Had ATC given us a different course, TCASII may have determined a no conflict. Because of TCASII RA command, we deviated assigned altitude. Supplemental information from acn 237053: climbed from 3000 ft to 4000 ft to avoid collision.

Original NASA ASRS Text

Title: ACR TAKES EVASIVE ACTION AS TCASII RA INDICATES A CLB, IN IMC.

Narrative: WHILE BEING RADAR VECTORED FOR ILS APCH, VFR TFC 500 FT BELOW OUR ACFT WAS NOT RPTED. OUR ACFT (LGT) WAS IMC, THE SINGLE ENG ACFT WAS JUST BELOW OVCST. APPARENTLY SINGLE ENG ACFT DRIFTED UP CAUSING TCASII TO COMPUTE A POSSIBLE COLLISION. HAD ATC GIVEN US A DIFFERENT COURSE, TCASII MAY HAVE DETERMINED A NO CONFLICT. BECAUSE OF TCASII RA COMMAND, WE DEVIATED ASSIGNED ALT. SUPPLEMENTAL INFO FROM ACN 237053: CLBED FROM 3000 FT TO 4000 FT TO AVOID COLLISION.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.